Process for producing crystals and the products thereof



Ap 1962 A. l- BENNETT, JR

PROCESS FOR PRODUCING CRYSTALS AND THE PRODUCTS THEREOF Filed Oct. 5, 1959 Fig.5.

Twin Plane *1 I54 I56) lss INVENTOR Allen I. Bennett ,Jr.

BY f ATTO NEY i l I I l 52 WITNESSES- ALB g United States Patent" 3,031,403 PRQCESS FOR PRGDUCING CRYSTALS AND THE PRQDUCTS THEREOF Allan I. Bennett, Jr., Pittsburgh, Pa., assignor to Westinghouse Electric (Iorporation, East Pittsburgh, Pa., a corporation of Pennsylvania Filed Oct. 5, 1959, Ser. No. 844,288 22 Claims. (Cl. 252-62.3)

the previously prepared crystahusually at the rate of the order of an inch an hour, whereby to produce a desired grown crystal member. It has been the invariable practice in this process to maintain the melt during the crystalgrowing process at a temperature slightly above the melting point of the solid material.

The nature and configuration of the withdrawn crystals produced by such prior art practices have generally been uncontrollable except within relatively broad limits. Thus, the thickness has not been readily maintained within precise dimensions. In many cases,surface and internal imperfections such as dislocations and other crystal structure flaws have been present in the grown crystals. In the semiconductor industry, crystals of silicon, germanium and compounds of the group III-group V elements have been grown from melts in accordance with this prior art practice. In order to employ such grown crystals in semiconductor devices, it has been necessary to saw them into slices using, for example, diamond saws. Thereafter, dice of desired shape have been cut from these slices. The sawed surfaces of'the dice have been lapped or otherwise mechanically polished to remove disturbed or otherwise unsatisfactory surface layers, which treatment is followed by an etch to remove microscopic surface imperfections. As a result of this Working, which is performed on expensive precision machinery and re quires highly-skilled labor, there may be a loss of as much as 90 percent of the original grown crystal in securing dice that have useful semiconductor shape and configuration.

The object of the present invention is to provide a process for producing crystals of precisely controllable thickness from supercooled melts of solid material.

A further object of the invention is to provide for growing fiat dendritic crystals from a supercooled melt of a semiconductor material while maintaining low temperature gradients in the crystals above the melt surface so that-imperfections in the grown crystal are minimized. Another object of the invention is to provide flat dendritic crystals with at least one twin plane extending therethrough of materials having a diamond cubic lattice structure with flat faces having precise (111) surfaces.

A further object of the invention is to provide a method for preparing dice from semiconductor materials without mechanical cutting operations by preparing fiat dendri'tic crystals from a supercooled melt of the material and, then, scoring the flat surfaces and breaking the flat crystals along the score lines to produce the desired dice.

A still further object of the invention is to provide doped fiat dendritic crystals of solid materials by preparing a melt of a material containing doping impurities in the proportions approaching those desired in the crystals, supercooling the melt and withdrawing therewith 3,031,403 Patented Apr. 24, 1932 dendritic crystals containing doping impurities in thedesired proportions.

Other objects of the invention will, in part, be obvious and will, in part, appear hereinafter.

For a better understanding of the nature and objects of the invention, reference should be had to the following detailed description and drawings, in which:

.FlGURE 1 is a view in elevation partly in section-of a crystal growing apparatus in accordance with the invention;

FIG. 2 is a greatly enlarged fragmentary view of a dendritic crystal having a single twin plane;

FIG. 3 is a greatly enlarged fragmentary view of a dendritic crystal having three twin planes;

FlG. 4 is a fragmentary view in elevation; and

FIG. 5 is an enlarged plan view of a dendritic crystal. in accordance with the present invention, it has been discovered that crystals of solid materials may be pre pared as flat dendritic crystals having a closely controllable thickness with relatively precise flat parallel faces. These flat dendritic crystals may be pulled or grown from melts of the material at a relatively high rate of speed of pulling of the order of times and greater than the linear pulling velocity previously employed in the art. The thickness of the crystals may be readily controlled and surface imperfections minimized or reduced by following the teachings of the present invention.

More particularly, in practicing the process, a melt of the material to be grown into a flat dendritic crystal is prepared at a temperature slightly above the melting temperature thereof. The surface of the melt is contacted with a previously prepared crystal having at least one twin plane at the interior thereof, the crystal being oriented with the 2l1 direction vertical to the melt surface. Other necessary or desirable crystallographic and physical features of the seed crystal will be pointed out in detail hereinafter. The seed crystal is dipped into the surface of the melt a sufiicient period of time to cause wetting. of the lower surface of the seed, usually a period of'tiine a few seconds to a minute is adequate, and, then, the melt is supercooled rapidly, following which the seed crystal is withdrawn with respect to the melt at a speed of the order of from one to ten inches a minute. Under some conditions, considerably slower pulling speeds than an inch per minute can be employed for example 0.2 inch per minute. Pulling speeds of from 10 to 25 inches per minute have given good results. The degree of supercooling and the rate of pulling can be readily so correlated that the seed crystal withdrawn from the melt comprises solidified melt material thereon of a precisely desired thickness and the desired crystallographic orientation. p

The present invention is particularly applicable to solid materials crystallizing in the diamond cubic lattice vstructure. Examples of such'materials are the elements silicon and germanium. Likewise, stoichiometric compounds having an average of four valence electrons per atom respond satisfactorily to the crystal growing process. Such compounds which have been processed with excellent results'comprise substantially equal molar proportions of an element from group III of the periodic table, and particularly aluminum, gallium and indium, combined with an element from group V of the periodic table, and particularly phosphorus, arsenic and antimony. Compounds comprising stoichiometric proportions of group II and group VI elements, for example, ZnSe and ZnS, can be processed. These materials crystallizing in the diamond cubic lattice structure are particularly satisfactory for various semiconductor applications. Furthermore, the diamond cubic lattice structure materials may be intrinsic or they may be doped with one or more impurities to produce n-type or p-type semiconductor materials. The crystal growing process of the present invention may be applied to all of these diiferent materials.

For a better understanding of the practice of the invention, reference should be had to FIG. 1 of the drawings wherein there is illustrated apparatus 10 for practicing the process. The apparatus comprises a base 12 carrying a support 14 for a crucible 16 of a suitable refractory material such as graphite to hold a melt of the material from which fiat dendritic crystals are to be drawn. Molten material 18, for example, germanium, is maintained withinthe crucible 16 in the molten state by a suitable heating means such, for example, as an induction heating coil 20 disposed about the crucible. Controls, not shown, are employed to supply an alternating electrical current to the induction coil 20 to maintain a closely controllable temperature in the body of the melt 18. The temperature should be readily controllable to provide a temperature in the melt a few degrees above the melting point and also to reduce heat input so that the temperature drops in a few seconds, for example in to seconds to a temperature at least one degree below the melting temperature and preferably to supercool the melt from 5 to'15 C., or lower. A cover 22 closely fitting the top of the crucible 16 may be provided in order to maintain a low thermal gradient above the top of the melt. Passing through an aperture 24 in the cover 22 is a seed crystal 26, preferably having three twin planes and oriented crystallographically as will be disclosed in detail hereinafter. The crystal 26 is fastened to a pulling rod 28 by means of a screw 39 or the like. The pulling rod 28 is actuated by suitable mechanism to control its upward movement at a desired uniform rate, ordinarily in excess of one inch per minute. A protective enclosure 32 of glass or other suitable material is disposed about the crucible with a cover 34 closing the top thereof except for an aperture 36 through which the pulling rod 28 passes.

Within the interior of enclosure 32 is provided a suitable protective atmosphere entering through a conduit 46 and, if necessary, a vent 42 may be provided for circulating a current of such protective atmosphere. Depending on the crystal material being processed in the apparatus, the protective atmosphere may comprise a noble gas such as helium or argon,'or a reducing gas such as hydrogen or mixtures of hydrogen and nitrogen, or nitrogen or the like or mixtures of two or more gases. In some cases, the space around the crucible may be evacuated to a high vacuum in order to produce crystals of ma terials free from any gases.

In'the event that the process is applied to compounds having one component with a high vapor pressure at the temperature of the melt, a separately heated vessel containing the component may be disposed in the enclosure 32 to maintain therein a vapor of such compound at a partial pressure sufficient to prevent impoverishing the melt or the grown crystals with respect to the component. Thus an atmosphere of arsenic may be provided when crystals of gallium arsenide are being pulled. The enclosure 32 may be suitably heated, for example, by an electrically heated cover, to maintain the walls thereof at a temperature above the temperature of the separately heated vessel containing the arsenic in order to prevent condensation of arsenic thereon.

Referring to FIG. 2 of the drawing, there is illustrated, in greatly enlarged view, a section of a seed crystal 26 having a single twin plane. Such seed crystal may be obtained in various ways, for example, by supercooling a melt of the solid material to a temperature at which a portion thereof solidifies, at which time some dendritic crystals having one or more internaltwin planes will be formed and may be removed from the melt. While these crystals may not be uniform, they are suitable for seed purposes. Also one can cut from a large twinned crys tal a section suitable for use as a seed crystal.

The seed crystal 26 comprises two relatively flat parallel faces 55) and 52' with an intermediate interior twin plane 54. The twin plane ordinarily will be precisely midway between the faces 50 and 52. Examination will show that the crystallographic structure of the preferred seed on both faces 50 and 52 is that indicated by the crystallographic direction arrows at the right and left faces, respectively, of the figure. It will be noted that the horizontal directions perpendicular to the fiat faces 50 and 52 and parallel to the melt surfaces are 111 The direction of growth of the dendritic crystal will be in a 211 crystallographic direction. If the faces 50 and 52 of the dendritic crystal 26 were to be etched preferentially to the [111] planes, they will both exhibit'equilateral triangular etch pits 56 whose vertices 58 will point upwardly while their bases will be parallel to the surface of the melt. It is an important feature of the preferred embodiment of the present invention that the etch pits on both faces 59 and 52 of seed crystal 26 have their vertices 53 pointing upwardly. A non-twinned crystal or a crystal containing two twin planes or any even number thereof will exhibit triangular etch pits on one face whose vertices will be pointing opposite to the direction of the vertices on the other face.

Referring to FIG. 3 of the drawing, there is illustrated a highly magnified portion of a seed crystal 126 which contains three twin planes 154, 156 and 158 extending across the entire cross-section thereof. The faces 1% and 152 have the same crystal orientation as the faces 50 and 52 of FIG. 2. The spacings or lamellae between the successive adjacent twin planes ordinarily are not uniform. The larnellar spacing, such as A between twin planes 154 and 156, and B between twin planes 156 and 158, is of the order of microns, that is from a fraction of a micron'to 15 to 20 microns or possibly greater. The ratio of A to B as determined from studies of numerous dendritic crystals has varied in the ratio of slightly more than 1 to as much as 18. Good seed crystals have been found to have lamellar spaces between successive twins of 5 microns and 1% microns, respectively. The desired lamellar spacing for good seed crystals is not my invention, but is the invention of John W. Faust, Jr. and Harold F. John, and is covered in their patent application Serial No. 16,384, filed March 21, 1960. In all cases all the twin planes in good seed crystals extend entirely through the seed. Where the twin planes terminate internally the seed crystal behaves as if no such twin plane is present insofar as pulling dendrites therewith from a melt.

It has been further discovered that, due to the microscopically small lamellar distances between twin planes, it is highly difiicult to determine whether one or more than one twin plane is present in a dendrite or seed crystal. In a number of cases, using all apparent care, it appeared that but a single twin plane was present in a given dendrite seed crystal. However, improved techniques have been developed which show clearly that these dendritic crystals contain three'or even more closely spaced twin planes. One of these improved techniques comprises scribing a line transverse of the length of the dendrite, bending the dendrite at the scribed line to bow it away from the scribed line until it fractures thereat, and, without polishing or otherwise working on the fractured face, examining it under a microscope at a magnification of at least x, and preferably 200 to 500x. The fracturing results in relatively flat faces developing at successive lamellae at different angles to each other which stand out distinctly under illumination. Also, preferentially etching of a polished cross-section, preferably cross-sections lapped at an angle to the flat face, so as to selectively distinguish the lamellae from each other, will enable the separate twin planes to be clearly distinguished.

The most satisfactory crystal growth is obtained by employing seed crystals of the type exhibited in FIG. 3 wherein three twin planes are present interiorly and are continuous across the entire cross-section of the seed.

Seed crystals having an odd number (other than 1 and 3', thatis', 5, 7- and up to 13' or more) twin planes containing the growth direction may be employed in practicing the process of this invention; due care being had to point the triangular etch pits on the outer faces of the crystal with their vertices upwardly and the bases parallel to the surface of the melt. Further, seed crystals containing an even number of twin planes may be employed for crystal pulling, though as desirable pulled crystals will not be obtainable as with the preferred three twin plane seed crystal as shown in FIG. 3. Normally, the pulled dendrite will exhibit the same twin plane structure as the seed crystal exhibits. Thus, the dendrite will have three twin planes extending through its entire length if the seed comprises three twin planes.

The direction of withdrawal of the seed crystal 26 having an odd numberof twin planes from the melt 18 must be with the direction of the vertices 53 of the etch pits being upward and the bases being substantially parallel to the surface of the melt. When so withdrawn, the melt will solidify at the bottom of the crystal in a satisfactory prolongation thereof; If the crystal 26 were to be inserted into the melt so that the vertices 58 pointed downwardly, very erratic grown crystals will be produced which are not only of non-uniform dimensions but grow at angles of 120 to the seed and produce very irregular spines, and generally are unsatisfactory.

When a relatively cold fiat seed crystalhas been introduced into the melt which is at a temperature of only a few degrees above the melting point of the material, the melt will dissolve the tip of the seed crystal. However, there will be a meniscus-like-contact between the seed crystal and the body of the melt. Such contact should be maintained by keeping the temperature of the melt close to the melting point of the material.

Upon reducing the power input to the heating coil in order to supercool the melt (or reducing the applied heat if other modes of heat application than inductive heating are employed) there will be observed in a period of time of the. order of 5 seconds after the heat input is reduced to a crucible of about 2 inches in diameter and length of 2 inches, the supercooling being about 8 C., an initial elongated hexagonal growth or enlargement on the surface of the melt at the tip of the seed crystal. The hexagonal surface growth increases in area so that in approximately 10 seconds after heat input is reduced its area is approximately 3 times that of the cross section of the seed crystal. At this stage, there will be evident spikes growing out of the two opposite hexagonal vertices lying in'the plane of the seed. These spikes appear to grow at the rate of approximately two millimeters per second. When the spikes are from two to three milimeters I low their melting point.

in length the seed crystal pulling mechanism is energized to pull the crystal from the melt at the desired rates. The initiation of pulling is timed to the appearance and growth of the spikes for best results.

After pulling the seed crystal upwardly from the supercooled melt, it will be observed that the flat solid diamond shaped area portion is attached to the seed crystal and that a downwardly extending dendritic crystal has formed at each end of the elongated diamond area adjacent the spike. Accordingly, two dendritic crystals can be readily pulled from the melt at one time from a single seed crystal. By continued pulling the two dendritic crystals may be extended to any desired length.

If the seed crystal is disposed so that one edge is nearer the thermal center of the melt crucible than is the other edge, it is possible to increase briefly either the pulling rate or the temperature of the melt, and under these variations the dendritic crystal furthest away from the thermal center or in a hotter region will usually stop growing and thereafter only a single dendritic crystal will be attached to and grow from the seed. Also, if the double dendritic crystal attached to the original seed crystal is introduced into the same or another melt slightly above the melting temperature and after supercooling the melt, on pulling the double dendritic crystal from appearance.

the surface, there will be formed two diamond shaped areas attached to the double dendrites and four dendritic crystals will be pulledtwo attached to each of the original dendrites. Thus, in one instance four germanium dendrites each 5 inches in length were pulled from the melt. While more than 4'dendritic crystals can be pulled from a melt, there may arise interference and other factors which will render such growth diflicult.

If the seed crystal 26 were to be pulled at a slowly increasing rate just as supercooling of the crucible is being effected by reducing the heat input, so that at the end of about 5 to 10 seconds the full pulling rate is being applied, then only one dendritic crystal will usually be attached to the seed crystal. I

The seed crystal need not be fiat. It may be of any suitable size or shape as long as its orientation corresponds to that shown in FIG. 2. Usually a portion of a previously grown dendritic crystal having twin planes will be quite satisfactory for use as a seed and ordinarily such will be used as the seed crystal. The pulled dendritic crystal need have no direct relation to the seed crystal as far as size is concerned. The pulled dendritic crystal will have a size and shape depending on the pulling conditions.

In growing satisfactory flat faced dendritic crystals in accordance with the present invention, the melts of the materials may be supercooled as much as 30 to 40 be- In practice, however, supercooling of from 5 to 15 C. has given best results with germanium and indium antimonide, for example. A greater degree of supercooling requires higher rates of crystal withdrawal from-the melt as well as requiring more precise control of the speed of pulling. Germanium and indium antimonide dendritic crystals have been satisfactorily pulled at rates of from 4 inches to 12 inches per minute from melts supercooled 5 C. to 15 C. As an example, these crystals have had a highly uniform thickness selected from the range of from 3 to 20 mils and a selected width of from 1 to 4 millimeters. The length of these crystals is limited solely by the pulling apparatus employed. Nodifficulty has been experienced in pulling crystals of, for example, --7 inches in length in a slightly modified crystal pulling furnace as normally used in the art.

Generally, the pulled dendritic crystals will have a thickness of the order of from 1 to 25 mils and the width across the flat faces may be from 20 mils to 200 mils and even wider. The surface at the flat faces will exhibit essentially perfect (111) orientation. The grown dendritic crystals of this invention will be essentially twinned crystals which are not of single crystal structure. Properly grown crystals will have faces that comprise flat areas on either side which are parallel and planar within a wavelength,*of..sodium light, per centimeter of length.

While the dendritic; crystals will exhibit some degree of edge serration, dendritic crystals have been obtained with fairly uniform edges having a minimum of ragged The serrated edges comprise only a small portion of the crystals and can be readily removed or left intact in dice since they do not affect the properties of the central or main body portion of the dendrites.

It has been discovered that when the dendritic crystals are grown under conditions where relatively cool gases come in contact with the dendritic crystal soon after it emerges from the melt, they will cool the grown crystal so as to produce a large temperature gradients while the crystal is in a plastic state and region of dislocations in the form of a narrow band along the center of the wide flat faces may appear. The values of harmful temperature gradients will be dependent in part on the cross-sectional area of the pulled crystal. However, temperature gradients of C. per centimeter and less are low enough for most crystals to be free from imperfections. It has been determined that this surface imperfection is due primarily to a high temperature gradient in the solid material' just above the melt which causes physical strains which affect the crystal'perfection of the plastic crystal. Such imperfections or dislocations may be minimized or completely eliminated by providing means for decreasing the temperature gradient in the newly drawn dendritic crystals for a distance above the surface of the melt, for example, a distance of the order of 1 cm. to 3 cm. Once the temperature of the crystals, for example, germanium has fallen to 700 C., there is no difiiculty due to cooling temperature gradients.

One means for producing such low temperature gradients is the application of a ceramic cap such as 22 to the top of the crucible whereby the heat of the melt is prevented from escaping and is radiated, back for an appreciable distance above the surface of' the melt. T us the radiant heat below the cover 22 in FIG. 1 prohibits the dendritic crystal as from cooling too rapidly or unevenly for an appreciable distance above the melt until the growing dendrite has a chance to cool below the range of plasticity without introducing dislocations and other structure imperfections. If desired, an external heating coil or sleeve may be disposed about the lower end of the dendrite crystal and an electrically conductive cap such as graphite applied about the crystal above the melt to be energized by high frequency current to produce a more controllable temperature gradient reducing effect.

By control of the temperature gradient at and near the melt'surface, twinned individual dentritic crystals have been grown from germanium and other materials which crystals have had surfaces of microscopic smoothness and crystallographic perfection. In some cases, only by interference pattern techniques can there be detected any change in the thickness of the surfaces, When examined, under the microscope and by interference pattern techniques usually the dentritic crystal faces will exhibit only flat steps, each of which is an essentially perfect, mirrorflat crystal surface. In some cases there will be only one or two steps per millimeter, the steps differing approximately by 50 angstroms in height. Over a length of 4 inches, the extreme variation in thickness in one crystal was less than 0.1 mil. It will be apparent that it is di. cult to'measure the difference in thickness of the grown crystal from point to point along its fiat faces due to the almost microscopic perfection thereof.

The present invention has been successfully employed in growing flat dentritic crystals of semi-conductor compounds such as indium antimonide and the =like. Crystals of these compounds were withdrawn from the melt at rates of the order of 5 inches per minute in the form of dendritic crystals have a width of approximately 3 millimeters and a thickness of 7 mils. The'surfaces were substantially flat and parallel throughout the entire length thereof except for the steps analogous to those in germanium.

It has been discovered that the fiat dendritic crystals of the present invention are relatively flexible, and crystals of a thickness of 7 mils may be bent on a radius of the order of 4 inches or even less without breaking. Consequently, crystals may be continuously drawn from the melt and Wound on a cylinder of a radius of this order in continuous lengths, as desired. The thinner crystals obviously can be wound to a smaller radius than crystals of greater thickness.

Referring to FIG. 4 of the drawing, there is illustrated a suitable mechanism for pulling a dendritic single crystal of indefinite length. The dendritic crystal 194 being withdrawn from melt 18 is disposed between rotating cylinders 160 and 102 flexibly mounted to grip the dendritic crystal between them and whose speed of rotation is correlated to 104 above the drums 160 and 102 may be coiled on a wide diameter spool or it may be severed from time to time into suitable lengths. Other means for continuously pulling the dendritic crystal from the melt may be employed.

It has been discovered that the dendritic crystal grows deep in the supercooled melt, and as shown in PEG. 4, probably has a wedge-shaped configuration 1% below the surface'of the melt. The grown dendritic crystals of the present invention have surfaces of such perfection that, in the case of semiconductor materials they may be employed for semiconductor applications simply by applying to the faces thereof desired alloys or solders without any intermediate polishing, lapping or etching. In fact, in general etching results in a degradation of the perfection of the crystal face. In all cases the crystal surfaces have a perfect (111) orientationas grown. For making such devices as diodes, transistors, photodiodes and other similar semiconductor devices, the (111) surfaces are a particularly desired orientation.

In accordance with this invention, it has been discovered further that doped crystals, particularly from semiconductor materials such as silicon, germanium and stoichiometric compounds of the group III-group V compounds, may be readily prepared. The dopingimpurities may comprise either adonor or acceptor material. Examples of donor impurities are antimony and phosphorus, while examples of suitable acceptor impurities are indium, gallium and aluminum. One noticeable advange obtained in practicing the present invention is that the previously known processes for growing crystals resulted in a crystal in which the proportions of the doping impurities were radically different from the proportions of the doping impurities in the melt. Contrary to previous crystal growing experience, it has been found in general the proportion of doping impurity in the pulled crystals will be much closer to the proportion in the same melt using the process of this invention. Thus gold will be present in germanium dendritic crystals in much higher proportions than in usual grown crystals produced by the slow pulling process of the prior art. Thus gold is rarely present in pulled germanium crystals in proportions in excess of l to the concentration of gold in the melt from which the crystal is grown whereas by the present invention dendritic crystals of germanium may be doped readily with gold in a ratio of 19 to the concentration of the gold present in the melt. It will be appreciated that the present process will result in much more convenient and more controllable doping than has been possible heretofore for many purposes.

' A further advantage of the present invention is that dice for semiconductor and other applications may be prepared from the grown dendritic crystals by a very simple operation which does not require the use of sawing, ultrasonic cavitation, or other involved cutting processes. To prepare a desired convex polygonal shape from a fiat dendritic crystal, it is only necessary to score the surface lightly with a diamond, for example, and upon a Y slight flexing, the dendrite will break along the score the supercooling of the melt 13 such that the desired thickness of dendritic crystal is withdrawn continuously. Dendrites in lengths of up to 30 feet have been grown in accordance with the invention. The portion of the crystal mark, thereby leaving the desired shape die.

Referring to :FIG. 5 of the drawing, there is illustrated a dendrite crystal 1434 which it is desired to cut into rec tangular dice. The fiat surface is scored lightly with a diamond to provide two parallel transverse grooves 69 and 62 defining therebetween a desired square die 66. The dendrite is placed on a flat surface with the end 64 beyond score mark 6t!v projecting beyond the edge of the surface and flexed lightly by applying a twcezer to the end and the end will break off at the score mark 60. In a similar manner, the die 66 is severed from the main body of the dendrite 164. The die 66 has a perfect (111) orientation and may be employed for semiconductor diode or transistor or similar device fabrication without any further mechanical working. it will be appreciated that the proceduresetforth herein will effect substantial econincreasing the pulling rate to 12 inches per minute.

omies as compared to the present day processes wherein expensive and time consuming diamond sawing, lapping and etching processes are employed with their attendant Waste of material and in many cases degradation of the semiconductor crystal. Theserrated edges 68 of the die need not be removed. The serrations have sides at 30 to the crystal axis. If it is desired to cut the die into several pieces or to trim the serrated edges, suitable scoring and flexing may be employed to do so.

The present process enables the growing or pulling of dendritic crystals from a melt at, for example, linear rates of from 100 to 1000 times greater than those employed heretofore. It will be understood that while only a single seed crystal 26 is illustrated in FIG. 1 as being pulled from the melt 18 a number of similar seed crystals may be employed and pulled simultaneously. By correlating the size and thickness of the crystal and the degree of supercooling of the melt with the rate of withdrawal the crystals may be pulled with substantially fiat faces of uniform thickness.

The following examples are illustrative of the present invention.

Example I In apparatus similar to FIG. 1, a graphite crucible containing a quantity of intrinsic germanium is heated by the induction coil to a temperature several degrees above the melting point of germanium, the temperature being about 938 C., until the entire quantity forms a molten pool. A dendritic seed crystal having at least one interior twin plane extending entirely therethrough and oriented as in FIG. 2 of the drawing, held vertically in a holder is lowered until its lower end touches the surface of the molten germanium. The contact with the molten germanium is maintained until a small portion of the end of the dendritic seed crystal has melted. Thereafter the temperature of the melt is lowered rapidly in a matter of 5 seconds by reducing current to the coil 20, to a tem perature 8 below the melting point of the germanium so scopic steps ditfering by about 50 angstroms and were of a quality suitable for semiconductor applications. Particularly good results were had where the seed crystals had three interior twin planes separated by distances of 5 microns and 1% microns respectively. The dendrite, of course, similarly had three twin planes.

The process of this Example I was repeated, except for The dendritic crystal was approximately 3.5 mils in thickness and of a width of about 30 mils. The surface perfection and flatness was exceptional. Thus in a length of an inch there was observed less than a wavelength of light variation in thickness. The faces of the dendritic crystal were of precisely (111) orientation.

Example II centrations were introduced into the germanium of this Example IIin lieu of the arsenic, and dendritic germanium crystals were grown therefrom. Doped germanium dendritic crystals with three twin planes and of a resistivity varying from approximately 1 to 30 ohm centimeters were produced in these several instances.

A dendritic crystal of a width of 2 millimeters, doped with aluminum and having a resistivity of approximately 15 ohm centimeters was produced in accordance with this Example II. It was scored with a diamond into lengths of between 2 and 3 millimeters, and upon flexing the crystal was severed into substantially rectangular dice. A layer of antimonytin solder was applied to one face of a die and upon fusion a p-n junction diode was obtained. Conductive leads were fastened to the opposite faces. The resulting diode exhibited good rectifying properties, thereby indicating the dendritic crystals were of satisfactory quality for semiconductor device applications.

In a similar manner silicon is melted, the molten silicon doped with indium, supercooled 5 C., and a doped dendritic crystal pulled.

' Example III A melt of indium antimonide was prepared following i the procedure of Example I employing apparatus as illustrated in FIG. 1 of the drawing. The indium'antimonide was withdrawn at the rate of 5 inches per minute from ,a melt supercooled 5 C. The resulting fiat dendrite crysthe precision of the orientation of pulled crystals disclosed herein.

It will be understood the above description and drawing are onlyillustrative and not limiting.

I claim as my invention:

1. In the process of producing thin flat crystals of a solid material crystallizing in the diamond cubic lattice structure selected from the group consisting of silicon, germanium, and stoichiometric compounds having an average of four valence electrons per atom, the steps comprising melting a quantity of the material, bringing the melt to a temperature slightly above the melting point of the material, contacting a surface of the melted material with a seed crystal of the material for a period of time to wet the seed crystal with the melted material, the seed crystal having a plural odd number of parallel interior twin planes, the crystal being oriented with a 111 direction parallel'to the surface of the melt and a 211 direction perpendicular to the surface of the melt, the

twin planes being parallel to the 211 direction, the dendritic crystal when etched exhibiting triangular etch pits on both faces with the vertices of the triangular etch pits being directed perpendicularly upward with respect to the melt and the bases of the etch pits being parallel to the melt surface, supercooling the melted material to a selected temperature, and pulling the seed crystal at a rate of the order of at least one inch a minute with respect to the melt surface while maintaining the selected temperature whereby the material from the melt solidifies on the seed crystal and produces an elongated flat dendritic crystal.

2. In the process of producing thin fiat crystals of a solid material crystallizing in the diamond cubic lattice structure selected from the group consisting of silicon,

germanium, and stoichiometric compounds having an average of four valence electrons per atom, the steps coml l melt to a temperature slightly above the melting point of the material, contacting a surface of the melted material with a seed crystal of the material for a period of time to wet the seed crystal with the melted material, the crystal having a plural odd number of parallel interior twin planes extending entirely through the seed crystal, the crystal having a lll direction parallel to the surface of the melt and a 2ll direction perpendicular to the surface of the melt, the twin planes being parallel to the 2l1 direction the dendritic crystal when etched exhibiting triangular etch pits on both faces with the vertices of the triangular etch pits being directed upward with respect to the melt and the base of each etch pit being parallel to the surface of the melt, supercooling the melted material to a selected temperature, and separating the seed crystal at a rate of the order of at least one inch a minute from the melt surface while maintaining the melt at the selected temperature, and maintaining a low temperature gradient in the withdrawn crystal for a short distance above the melt surface so as to minimize imperfections, whereby the material from the melt solidifies on the seed and produces an elongated flat dendritic crystal substantially free from imperfections.

3. In the process of producing thin fiat crystals of germanium, the steps comprising preparing a molten body of germanium at a temperature slightly above its melting. point, contacting the surface of the molten germanium with a seed crystal of germanium, the seed crystal having a plural odd number of parallel interior twin planes extending entirely through the seed crystal, the seed crystal having a lll direction parallel to the surface of the melt and a 2ll direction perpendicular to the surface of the melt, the twin planes being parallel to the 211 direction the seed crystal when etched exhibiting triangular etch pits on both faces with the vertices" of the triangular pits being directed upwardly and the base of the triangle being parallel to the surface of the melt, rapidly supercooling the molten germanium at least one degree centigrade to a selected temperature, and withdrawing the dendritic seed crysal with respect to the melt at a rate of the order of at least one inch a minute while maintaining the selected supercooled temperature, the rate of withdrawal being correlated to the degree of supercooling so that the germanium from the melt solidifies on the seed crystal in the form of a flat dendritic crystal.

4. In the process of producing thin flat crystals of germanium, the steps comprising preparing a molten body of germanium at a temperature slightly above its melting point, contacting the surface of the molten germanium with a seed crystal of germanium, the seed crystal having a plural odd number of parallel interior twin planes extending entirely through the seed crystal, the crystal having a lll direction perpendicular to the surface of the melt and a 21l direction perpendicular to the surface of the melt, the twin planes being parallel to the 2ll direction, the seed crystal when etched exhibiting triangular etch pits on both faces with the vertices of the triangular pits being directed upwardly and the base of the triangles being parallel to the surface of the melt, rapidly supercooling the molten germanium at least one degree centigrade to a selected temperature, and withdrawing the dendritic seed crystal with respect to the melt at a rate of the order of from one inch to ten inches a minute while maintaining the selected temperature, the rate of withdrawal being correlated to the degree of supercooling so that the germanium from the melt solidifies on the seed crystal in the form of a flat dendritic crystal and applying means to prevent excessive heat loss from the withdrawn germanium crystal for a short distance above the melt surface so as to maintain a low temperature gradient therein and thereby to reduce crystal imperfections.

5'. In the process of producing thin flat crystals of silicon, the steps comprising preparing a molten body of silicon at a temperature slightly above its melting point, contacting the surface of the molten silicon with a seed crys ml of silicon, the seed crystal having an interior twirl plane extending entirely through the seed crystal, the crystal having a lll direction parallel to the surface of the melt and a 21l direction perpendicular to the surface of the melt, the seed crystal when etched exhibiting triangular etch pits on both faces with the vertex of the triangular pits being directed upwardly, supercooling the molten silicon at least one degree centigrade, and withdrawing the dendritic seed crystal with respect to the melt at a rate of the order. of from one to ten inches a minute, the rate of withdrawal being correlated to the degree of supercooling so that the silicon from the melt solidifies on the seed crystal in the form of a fiat dendritic crystal.

6. In the process of producing thin fiat crystals of silicon, the steps comprising preparing a molten body of silicon at a temperature slightly above its'melting point, contacting the surface of the molten silicon with a seed crystal of silicon, the seed crystal having an interior twin plane extending entirely through the seed crystal, the seed crystal having a lll direction parallel to the surface of the melt and a 21l direction perpendicular to the surface of the melt, the seed crystal when etched exhibiting triangular etch pits on both faces with the vertices of the triangular pits being directed upwardly and their bases parallel to the surface of the melt, supercooling the molten silicon at least one degree centigrade, and withdrawing the seed crystal with respect to the melt at a rate of the order of from one to ten inches a minute, the rate of withdrawal being correlated to the degree of supercooling so that the'silicon from the melt solidifies on the seed crystal as a flat dendritic crystal, and applying means to the withdrawn silicon crystal for a short distance above the melt surface to prevent excessive heat losses so as to maintain a low temperature gradient therein thereby to reduce crystal imperfections and dislocations.

7. In the process of producing fiat crystals of an essentially stoichiometric compound of at least one element selected from the group consisting of aluminum, gallium and indium and at least one element from the group consisting of phosphorus, arsenic, and antimony, the steps comprising preparing a molten body of the compound at a temperature slightly above the melting point, contacting the surface of the molten body with a seed crystal of the compound, the seed crystal having an odd number of interior twin planes extending entirely through the seed crystal, the seed crystal having a lll direction parallel to the surface of the melt and a 2ll direction perpendicular to the surface of the melt, the seed crystal when etched exhibiting triangular etch pits on both faces with the vertices of the triangular etch pits being directed upwardly with respect to the melt and the bases being parallel to the surface of the melt, supercooling the melt at least one degree centigrade, withdrawing the seed crystal with respect to the surface of the molten body at a rate of the order of from one to ten inches per minute, whereby molten compound solidifies on the seed crystal as it is withdrawn to produce an elongated fiat dendritic crystal.

8. In the process of producing thin flat doped crystals of a solid material crystallizing in the diamond cubic lattice structure, the steps comprising preparing a melted body of the material, adding a doping material to the melted body in proportions relatively closely equal to the proportions desired in the doped crystals, bringing the melt to a temperature slightly above the melting point of the material, contacting a surface of the melted mate'- rialtwith a seed crystal of the material for a period of time to wet the seed crystal with the melted material, the seed crystal having a plural odd number of parallel interior twin planes extending entirely through the seed crystal, the seed crystal having a lll direction parallel to the surface of the melt and a 21l direction perpendicular to the surface of the melt, the twin planes being parallel to the 2ll direction, the seed crystal when etched exhibiting triangular etch pits on both faces with the vertices of the triangular etch pits being directed perpendicularly upwardly with respect to the melt and the bases being parallel to the surface of the melt, supercooling the melted material to a selected temperature, separating the seed crystal at a rate of the order of from one to ten inches per minute from the melt surface while maintaining the selected melt temperature, whereby the material from the melt solidifies thereon and produces an elongated flat dendritic crystal, the resulting fiat crystal having the doping material distributed therein in the desired proportions.

9. In the process of producing flat dice from a solid material crystallizing in the diamond cubic lattice structure, the steps comprising preparing a melted body of the material, bringing the melt to a temperature slightly above the melting point of the material, contacting a surface of the melted material with a flat dendritic seed crystal of the material for a period of time to wet the seed crystal with the melted material, the seed crystal having a plural odd number of parallel interior twin planes extending entirely through the seed crystal, the seed crystal having a l11 direction parallel to the surface of the melt and a 2ll direction perpendicular to the surface of the melt, the twin planes being parallel to the 2l1 direction, the seed crystal when etched exhibiting triangular etch pits onlboth faces with the ver-. tices of the triangular etch pits being directed perpendicularly upwardly with respect to the'melt, supercooling the melted material to a selected temperature, and separating the seed crystal at a rate of the order of from one to ten inches per minute from the melt surface while maintaining the selected melt temperature whereby the material from the melt solidifies thereon and produces an elongated flat dendritic crystal, scoring the surface of the resulting flat dendritic crystal into selected sized areas and breaking the crystal at the score lines to produce the desired dice.

10. In the process of producing thin flat crystals of a solid material crystallizing in the diamond cubic lattice structure selected from the group consisting of silicon, germanium, and stoichiometric compounds having an average of four valence electrons per atom, the steps comprising melting a quantity of the material, adding a doping material to the melted material, bringing the melt to a temperature slightly above the melting point of the material, contacting a surface of the melted material with a seed crystal of the material for a period of time to wet the seed crystal with the melted material, the seed crystal having a plural odd number of parallel interior twin planes, the crystal being oriented with a 111 direction parallel to the surface of the melt and a 211 direction perpendicular to the surface of the melt, the twin planes being parallel to the 211 direction, the dendritic crystal when etched exhibiting triangular etch pits on both faces with the vertices of the triangular etch pits being directed perpendicularly upward with respect to the melt and the bases of the-etch pits being parallel to the melt surface supercooling the melted ma-' terial to a selected temperature, and pulling the seed crystal at a rate of the order of at least one inch a minute with respect to the melt surface while maintaining the selected temperature whereby the material from the melt solidifies thereon and produces an elongated flat dendritic doped crystal.

11. In the process of producing thin flatdice from crystals of silicon, the steps comprising preparing a molten body of silicon at a temperature slightly above its melting point, adding a doping material to the molten silicon, contacting the surface of the molten silicon with a seed crystal of silicon, the seed crystal having a 14 to the surface of the melt, the twin planes being parallel to the 211 direction, the seed crystal when etched exhibiting triangular etch pits on both faces with the vertices of the triangular pits being directed upwardly, and the bases being parallel to the surface of the melt, supercooling the molten silicon at least one degree centigrade, and withdrawing the seed crystal with respect to the melt at a rate of the order of from one to ten inches per minute, the rate of withdrawal being correlated to the degree of supercooling so that the silicon from the melt solidifies on the seed crystal as a flat dendritic doped crystal, scoring the suface of the resulting flat dendritic crystals into selected size areas and breaking the crystal at the score plural odd number of parallel. interior twin planes, the

seed crystal having a 11l direction parallel to the surface of the melt and a 211 direction perpendicular lines to produce the desired dice. I

I 12.lA n elongated dendritic crystal of a solid material crystallizing in the diamond cubic lattice structure selected from the group consisting of silicon, germanium and stoichiometric compounds having an average of four valence electrons per atom, the dendritic crystal being of a thickness of from 1 to 25 mils and a width of from 20 to 200 mils, and having a plurality of interior twin planes extending entirely therethrough, and two fiat exterior surfaces parallel to the twin planes comprising a series of flat parallel faces having a high degree of planarity of the order of a wavelength of light, each face differing from adjacent faces in a step of the order of 50 angstroms, and the surface of the material at the faces having essentially perfect (111) orientation, the vdendritic crystal being substantially free of imperfections at the flat surfaces throughout its length and being sutficientlyf uniform for use along substantially its entire length for semiconductor devices.

13. An elongated dendritic crystal of silicon having a plural odd number of parallel twin planes extending entirely therethrough and having fiat parallel surfaces of a width of from 20 to 200 mils and of a thickness of from 1 to 25 mils, the surfaces being parallel to, the twin planes and comprising a series of flat faces, the faces having a planarity of the order of a wavelength of light, each face differing from adjacent faces in a step of the order of 50 angstroms, the surface of the material at the faces having essentially perfect (111) orientation, the dendrite being substantially free of imperfections at the flat surfaces throughout its length and being sufficiently uniform forusealong its entire length for semiconductor devices. r

14. An elongated'dendritic crystal of germanium having aplural odd number of parallel twin planes extending entirely through the dendritic crystal having flat parallel surfaces of a width of from 20 to 200 mils and of a thickness of from 1 to 25 mils, the fiat surfaces being parallel to the twin planes and comprising a series of fiat faces, the faces having a planarity of the order of a wavelength of light, each face differing from adjacent faces in a step of the order of 50 angstroms, the surface of the material at the faces having essentially perfect (111) orientation, vthe dendritic crystal being substantially free of imperfections .at the flat surfaces throughout its length and being sufficiently uniform for use along substantially its entire length for. semiconductor devices.

'15. A dendritic crystal of a stoichiometric compound of at least one element selected from the group consisting of aluminum, gallium and indium and at least one element selected from the group consisting of phosphorus, arsenic and antimony, the crystal having a plural odd number of parallel twin planes and having flat parallel surfaces of a width of from 20 to 200 mils and of a thickness of from 1 to 25 mils, the flat surfaces being parallel to the twin planes and comprising a series of flat faces, and faces having a planarity of the order of a wavelength of light; each face diifering from adjacentfaces in a step of the order of 50 angstroms the surface of the material at the faces having essentially perfect (111) orientation, the dendritic crystal being substantially free of imperfections at the flat surfaces throughout its length and being sufficiently uniform for use along substantially its entire length for semiconductor devices.

16. An elongated dendritic crystal of a solid material crystallizing in the diamond cubic lattice structure, the material being doped with an impurity selected from the group consisting of donor and acceptor impurities, the crystal having a plural odd number of parallel interior twin planes, the crystal having a thickness of from 1 to 25'mils,the crystal having flat exterior surfaces parallel to the twin planes and comprising a series of flat faces, the faces of the crystal being substantially fiat and parallel within a wavelength of light, each face differing from adjacent faces in a step of the order of 50 angstroms, the faces having essentially perfect (111) orientation and free from any mechanical working, the dendritic crystal being substantially free of imperfections at the flat surfaces throughout its length and being sufficiently uniform for use along substantially its entire length for semiconductor devices.

17. A dendritic crystal of a solid material crystallizing in the diamond cubic lattice structure selected from the group consisting of silicon, germanium and stoichiometric compounds having an average of four valence electrons per atom, the dendritic crystal being of a thickness of from 1 to 25 mils and a width of from to 200 mils, and having three parallel twin planes extending entirely therethrough, and two fiat exterior surfaces parallel to the twin planes and comprising a series of flat parallel faces having a high degree of'planarity of the order of a wavelength of light, each face differing from adjacent faces in a step of the order of 50 angstroms, the surface of the material at the faces having essentially perfoot (111) orientation, the dendritic crystal being substantially free of imperfections at the flat surfaces throughout its length and being sufficiently uniform for use along substantially its entire length for semiconductor devices.

'18; In the process of growing thin flat dendritic crystals of a material crystallizing in the diamond cubic lattice structure from a melt of said materials, the steps comprising bringing a seed crystal having at least two parallel interior twin planes in the 211 direction extending entirely therethrough in contact with the melt so that it is wetted by the melt, the dendritic crystal when etched exhibiting triangular etch pits on both faces with the vertices of the etch pits directed in the direction of growing of the dendritic crystals, supercooling the melt to a selected temperature and pulling the seed crystal with respectto the melt while maintaining the selected temperature whereby material from the melt solidifies thereon to produce an elongated fiat dendritic crystal.

19. In the process of growing thin flat dendritic crystals of a material crystallizing in the diamond cubic lattice structure from a melt of said materials, the steps comprising bringing aseed crystal-having a plural odd number of interior twin planes extending entirely therethrough the planes extending in the 2ll direction in contact with the melt so that it is wetted by the melt, the dendritic crystal when etched exhibiting triangular etch pits on both faces with the vertices of the etch pits directed in. the direction of the pulling of the dendritic crystals, supercooling the melt to a selected temperature and pulling the seed crystal with respect to the melt while maintaining the selected temperature whereby material from the melt solidifies thereon to produce an elongated flat dendritic crystal and maintaining a low temperature gradient of the order of 100 C. per centimeter and less in the grown dendritic crystal for a short distance away from the melt until it has cooled to a temperature where the dendrite crystal is no longer plastic so that imperfections due to thermal strains are greatly reduced.

20. In the process of growing thin flat dendritic crystals of a'rnaterial crystallizing in the diamond cubic lattice structure from a melt of said material, the steps comprising bringing a seed crystal having three parallel 1nter1or twin planes extending entirely therethrough in contact with the melt so that it is wetted by the melt, the three twin planes being substantially perpendicular to the surface of the melt, the dendritic crystal when etched exhibiting triangular etch pits on bothfaces with the vertices of the etch pits directed in the direction of pulling of the dendritic crystal, supercooling the melt to a selected temperature and pulling the seed crystal with respect to the melt while maintaining the selected temperature whereby material from the melt solidifies-thereon to produce an elongated flat surfaced dendritic crystal.

21. In the process of growing thin flatdendritic crystals of a material crystallizing in the diamond cubic lattice structure from a melt of said material, the steps comprising bringing a seed crystal having three parallel interior twin planes extending entirely therethrough in contact with the melt so that it is wetted by the melt, the dendritic crystal when etched exhibiting triangular etch pits on both faces with with the vertices of the etch pits directed in the direction of pulling of the dendritic crystal, supercooling the melt to a selected temperature and pulling'the seed crystal in a 2l1 direction parallel to the twin planes with respect to the melt while maintaining the selected temperature, whereby material from the melt solidifies thereon to produce an elongated fiat dendritic crystal and maintaining a low temperature gradient of the order of C. per centimeter and less in the grown dendritic crystal for a short distance away fromthe melt until it has cooled to a temperature where the dendrite crystal is no longer plastic so that imperfections due to thermal strains are greatly reducedj v 22. In the process of producing thin flat crystals of a solid material crystallizingin the diamond cubic lattice structure selected from the group consisting of silicon, germanium, and stoichiometric compounds having an average of four valence electrons per atom, the steps comprising melting a quantity of the material, bringing the melt to a temperature slightly above the melting point of the material, contacting a surface of the melted material with a seed crystal of the material for a period of time to wet the seed crystal with the melted material, the crystal having three parallelinterior twin planes extending entirely through the seed crystal, the twin planes being parallel to 2l-1 direction, the crystal having a l1l direction parallel to the surface of the melt and a 211 direction perpendicular to the surface of the melt, the dendritic crystal when etched exhibiting triangular etch pits on both faces with the vertices of the triangular etch pits being directed upwardly with respect to the melt and the base of each etch pit being parallel to the surface of the melt, supercooling the melted material to a selected temperature, and separating the seed crystal at a rate of the order of at least one inch 2. minute from the melt surface while maintaining the selected supercooled temperature, whereby the material from the melt solidifies thereon and produces an elongated flat dendritic crystal, and maintaining a low temperature gradient in the withdrawn dendritic crystal for a short distance above the melt surface so as to minimize imperfections.

OTHER REFERENCES Canadian Journal of Physics, vol. 34, January to June 1956, pages 234 to 240 and 4 pp. of plates. 

1. IN THE PROCESS OF PRODUCING THIN FLAT CRYSTALS OF A SOLID MATERIAL CRYSTALLIZING IN THE DIAMOND CUBIC LATTICE STRUCTURE SELECTED FROM THE GROUP CONSISTING OF SILICON, GERMANIUM, AND STOICHIOMETRIC COMPOUNDS HAVING AN AVERAGE OF FOUR VALENCE ELECTRONS PER ATOM, THE STEPS COMPRISING MELTING A QUANTITY OF THE MATERIAL, BRINGING THE MELT TO A TEMPERATURE SLIGHTLY ABOVE THE MELTING POINT OF THE MATERIAL, CONTACTING A SURFACE OF THE MELTED MATERIAL WITH A SEED CRYSTAL OF THE MATERIAL FOR A PERIOD OF TIME TO WET THE SEED CRYSTAL WITH THE MELTED MATERIAL, THE SEED CRYSTAL HAVING A PLURAL ODD NUMBER OF PARALLEL INTERIOR TWIN PLANES, THE CRYSTAL BEING ORIENTED WITH A <111> DIRECTION PARALLEL TO THE SURFACE OF THE MELT AND A <211> DIRECTION PERPENDICULAR TO THE SURFACE OF THE MELT, THE TWIN PLANES BEING PARALLEL TO THE <211> DIRECTION, THE DENDRITIC CRYSTAL WHEN ETCHED EXHIBITING TRIANGULAR ETCH PITS ON BOTH FACES WITH THE VERTICES OF THE TRIANGULAR ETCH PITS BEING DIRECTED PERPENDICULARLY UPWARD WITH RESPECT TO THE MELT AND THE BASES OF THE ETCH PITS BEING PARALLEL TO THE MELT SURFACE, SUPERCOOLING THE MELTED MATERIAL TO A SELECTED TEMPERATURE, AND PULLING THE SEED CRYSTAL AT A RATE OF THE ORDER OF AT LEAST ONE INCH A MINUTE WITH RESPECT TO THE MELT SURFACE WHILE MAINTAINING THE SELECTED TEMPERATURE WHEREBY THE MATERIAL FROM THE MELT SOLIDIFIES ON THE SEED CRYSTAL AND PRODUCES AN ELONGATED FLAT DENDRITIC CRYSTAL. 